AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Stepwise Termination of the Permian Reef Complex, Permian Basin: Implications for Structural and Depositional Patterns of Saline Giants
(1) Dept. of Geological Sciences, University of Texas at Austin, Austin, TX.
(2) Bureau of Economic Geology, University of Texas at Austin, Austin, TX.
Carbonate-rimmed evaporite-filled basins are a common configuration in major oil and gas reservoir systems such as in the Paradox, Permian, Michigan, and Pre-Caspian basins. These petroleum systems are generally well-understood, but the driving force for the carbonate-to-evaporite transition is not. The Late Guadalupian Capitan Reef and its equivalent shelf (Tansill) and basinal (Lamar, post-Lamar siltstone) strata provides a unique depositional and structural record of cessation and collapse of this long-lived Permian carbonate platform and subsequent basinal evaporite filling.
Mapping of the Tansill-Capitan-Lamar system using photomosaic and airborne lidar was conducted in Dark, Walnut, and Rattlesnake Canyons, with reconnaissance mapping of Slaughter and McKittrick Canyons. This provides basic data on shelf-slope facies composition and architecture as well as location, spacing and timing of syndepositional fractures and collapse scars. The transition from a healthy Capitan platform to Castille evaporite deposition includes: (1) a change in shelf-margin trajectory from low-angle progradation to retrogradation and aggradation; (2) evolution from a healthy carbonate shelf with a well-developed periplatform mud apron (Lower Tansill-Capitan-Lamar) to a microbially-dominated shelf and shelf-margin with only slope breccias and no mud export to form a periplatform blanket (Upper Tansill-Capitan-post-Lamar siltstone); (3) oversteepening of the shelf-margin reef, collapse events, and the only well-developed suite of syndepositional marine-cemented fracture systems known from the Capitanian system; (4) a complete turn-over of outer shelf fauna with the replacement of the suite of larger fusuline forams by smaller more diverse forms, and a proliferation of microbial carbonates, and (5) deposition of a clast-rich onlapping debris wedge restricted to the proximal slope and complete starvation on basin floor prior to onset of evaporite deposition.
This record favors a Late Capitanian sea-level rise that drove shelf-margin aggradation, rimming, and isolation of the basin rather than the oft-speculated eustatic fall. Restriction led to the evolution of a microbially-dominated margin with little or no sediment export to the slope and basin and eventual oversteepening and failure of the margin. A similar aggradational stacking of shelf-margin reefs is also observed immediately prior to onset of Messinian evaporites in the Mediterranean and possibly in the Pre-Caspian.